In general, a conventional microwave amplifier is known from, for example, Japanese Patent Laid-open No. 285811/1986. This first prior art will how be explained with reference to FIG. 21.
In this drawing, reference numeral 110 shows a microwave amplifier; reference numeral 111 indicates a field-effect transistor (will be referred to as an "FET"); reference numeral 112 represents a resistor; and reference numeral 113 shows a connection conductor for connecting the FET 111 to the ground plane. This connection conductor 113 functions as a tip shortcircuit stub which contains an inductance component in a high frequency field. Also, reference numeral 114 represents a tip open stub, the length of which is equal to a 1/4 wavelength at an operating frequency of the circuit. Specifically, the 1/4 wavelenghth means a 1/4 of a wavelength. The wavelength corresponds to the operating frequency of the circuit.
As indicated in FIG. 22, since the connection conductor 113 owns the inductance component "A", the FET 111 can be grounded by employing only the connection conductor 113 at a low frequency under which the inductance component "A" is negligible. However, the inductance component "A" is not negligible in a high frequency, and thus, a gain of the FET 111 is deteriorated. As a consequence, it is possible to avoid such a gain deterioration of the FET 111 by employing the tip open stub 114 so as to ground the FET 111 in the high frequency sense. Also, at this stage, in order to suppress unwanted oscillation of the FET 111, the resistor 112 is connected.
Next, a description will now be made of a second prior art with reference to FIG. 23. FIG. 23 is the diagram for representing the low noise amplifier described in "HEMT DIRECT COOLING TYPE LOW NOISE AMPLIFIER", SHINGAKU GIHO MW 92-149,
Japanese Electronic Information Communication Institute. In this drawing, reference numeral 120 indicates a low noise amplifier; reference numeral 121 represents an FET; and reference numeral 122 shows an inductor. A source electrode of the FET 121 is grounded via the inductor 122. As a result, the input impedance for minimizing noise of the FET 121 can be located in the vicinity of the impedance for minimizing the reflection, and also both the noise figure and the reflection coefficient on the input side can be improved at the same time.
On the other hand, any of the above-described prior art owns the below-mentioned problems. First, the microwave amplifier 110 corresponding to the first prior art owns such a problem that unnecessary electric power is consumed by the resistor 112 connected to the source electrode of the FET 111, and thus, the output power characteristic would be deteriorated. Also, as shown in FIG. 24, the low noise amplifier 120 corresponding to the second prior art owns such a problem that since the inductor 122 connected to the source electrode of the FET 121 owns the stray capacitance "B", this inductor is resonated at a predetermined frequency, and thus, operation would become unstable.
The present invention has an object to solve these problems.